The Natural History of "Big Night"

On the first rainy night of spring, when the temperature is about 45-50°F, hundreds of thousands of these creatures leave their underground winter burrows and make the journey to their birthplace.

This mass migration of amphibians each spring is a naturally recurring event known as Big Night. People from all over the Northeast will travel to watch this phenomenon, often stopping traffic to allow hundreds of salamanders or frogs to cross roads in pouring rain.

An Annual Journey

During this migration, these creatures will travel (sometimes up to a half-mile) through the woods to a large pool that has formed from snow-melt called a vernal pool. They will then spend a few days or weeks at the vernal pool courting mates through song and—in the case of salamanders—elaborate courtship dances.

This mass courtship ritual is known as congressing. From the high-pitched trill of a Spring Peeper to the quacking sound of the Wood Frog, the males use their calls to attract their mates. The females lay eggs in the pools, and eventually the adults return to their life in the forest.

Evolutionary Advantage

Wood Frog (Photo: Tom Lautzenheiser)

Why are the salamanders and frogs compelled to get up during the still-cool early spring? On some instinctual level, they know the vernal pools will not last forever and will dry up by late summer—a big problem for amphibians, whose early life cycles require water.

When their eggs hatch, they release the young salamanders (as larvae) and frogs (as tadpoles) into the water, where they will remain until they develop the lungs necessary to live on land. And all of this needs to happen before the vernal pool dries up.

But the short life of the vernal pool is a blessing for the young amphibians since they have fewer predators to worry about in the water. And as the temperatures continue to warm up, more amphibians will join the gatherings at the vernal pools. American Toads breed in April, followed by bull frogs and green frogs once the temperature of the water reaches 70°F.

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